This invention relates to continuously variable transmission and more particularly to starting devices for toroidal transmissions.
Toroidal transmissions are either half toroidal or full toroidal traction drives which typically utilize dual cavities for maximum efficiency. The dual cavity units have two input discs and two output discs each having a toroidal or partial toroidal shape. The output discs are typically located centrally between the input discs. Each input disk is engaged with a respective output disc through a plurality of traction rollers. The angle of the rollers is varied to change the drive ratio between the input and output discs. The dual cavity continuously variable unit (CVU) requires either a countershaft or a split torque arrangement to transmit power from the center (output) discs.
Half toroidal CVUs can use an input starting device, however, full toroidal CVU employ a geared neutral arrangement to effect vehicle launch. One example of a geared neutral arrangement is shown in U.S. Pat. No. 5,607,372 issued to Lohr on Mar. 4, 1997. This patent describes a half toroidal CVU having a coaxial split torque arrangement having a planetary carrier and two sun gear members. The carrier is the CVU input member and one of the sun gears is driven by the CVU output member. The other sun gear (output) is drivingly connected to a planetary gear assembly. By changing the roller angle in one direction, from neutral, a forward output is achieved and by changing the roller angle in the other direction, from neutral, a reverse output is achieved. This avoids the need for a starting device.
It is an object of the present invention to provide an improved continuously variable transmission (CVT) having a full toroidal CVU and an output starting clutch.
In one aspect of the present invention, a split torque planetary gear set is combined with a full toroidal CVU to provide a forward variable ratio range and a reverse fixed ratio. In another aspect of the present invention, the CVU has input members directly driven by a prime mover and the planetary gear set has a planet carrier member driven by the prime mover and a sun gear member driven by the output member of the CVU. In another aspect of the present invention, the planetary gear set has two ring gear members that rotate in opposite directions (relative to each other) when the CVU is set at a maximum underdrive ratio.
In yet another aspect of the present invention, a selectively operable mechanical clutch is disposed between the ring gear members and a selectively engageable starting clutch to provide a forward output and a reverse output from the CVU to the starting clutch. In still another aspect of the present invention, the ratio of the sun gear member to one ring gear member establishes a first directional output between the prime mover and a transmission output shaft driven by the starting clutch, and the ratio between the sun gear member and the other ring gear member establishes a second directional output, opposite the first directional output, between the prime mover and the transmission output shaft.
In a further aspect of the present invention, the CVU ratio remains constant during reverse operation and is varied during forward operation. In a yet further aspect of the present invention, the CVU is maintained at a maximum underdrive ratio during reverse operation and varied between the maximum underdrive ratio and a maximum overdrive ratio during forward operation. In a still further aspect of the present invention, the vehicle speed in forward operation is changed by either or both CVU ratio and prime mover speed, and the vehicle speed in reverse operation is changed by prime mover speed only. In a yet still further aspect of the present invention, the starting clutch is engaged to launch the vehicle in both the forward and the reverse directions.
This invention uses a CVU and a coaxial planetary gear set with a xe2x80x9csplit torque arrangementxe2x80x9d. A first ring gear member to sun gear member ratio is selected to provide the desired maximum overdrive ratio in the backwards direction (opposite engine rotation) of the CVU. This will increase the transmission overall ratio to approximately twice the CVU overall ratio, which will greatly improve the ability of the starting clutch to launch the vehicle at maximum underdrive. Selecting a backwards output direction of rotation, for forward drive operation, allows a design with minimum content and higher efficiency than the forward output designs. Reverse is achieved by adding a second ring gear member to provide a speed ratio equal in magnitude (with opposite direction) to the lowest forward CVT speed ratio. When the torque is output through the second ring gear member, the transmission could be used as a geared neutral CVT, however, the torque capacity and efficiency will be lower than the path provided by the first ring member.
The backward transmission output shaft rotation requires a xe2x80x9cbackwardsxe2x80x9d hypoid to provide a forward vehicle direction of travel. The hypoid gears used on the front axle of today""s four wheel drive vehicles are generally designed to produce maximum efficiency with the opposite xe2x80x9cprop shaftxe2x80x9d rotation. Therefore, placing a xe2x80x9cfrontxe2x80x9d hypoid in the rear will provide the correct gear geometry for maximum efficiency with reverse prop shaft rotation; however, the wheel direction of rotation will not be correct. The front hypoid will need to be rotated 180 degrees about the prop shaft (i.e. installed upside down) to provide the correct wheel direction of rotation.
The proposed gear arrangement minimizes spin losses by using a manual transmission type dog clutch (with synchronizers) to eliminate the cost and spin losses of a second starting clutch. The synchronizers will only have to accelerate the inertia of the inner clutch plates and hub when shifting between forward and reverse. This inertia is significantly lower than the inertia of the driven disk and input shaft of a conventional manual transmission. The synchronizer can be activated by a conventional mechanical mechanism that is attached to the PRNDL lever, not shown, for minimum cost, or it can be activated hydraulically by any of the well-known electro-hydraulic control systems.